SUMMARY: Glioblastoma (GBM) is an aggressive cancer with a dismal prognosis (median survival ~15 mo). Although antibody-drug conjugates (ADC) hold great promise, this approach is unexpectedly ineffective in a large number of patients. Genomic analyses have identified alterations in multiple genes that regulate endocytic pathways in GBM tumors but the possibility that reconfiguration of the endocytic pathways can compromise the efficacy of ADC therapy has not been explored. The goal of the proposed study is to test the hypothesis that tumor-associated reconfiguration of endocytic trafficking in perivascular cancer stem-like cells (CSLCs) influences the outcome of ADC therapies in GBM. To test this hypothesis, we are utilizing a highly collaborative transdisciplinary approach and focusing on ABT-414 as the prototypic ADC. ABT-414 is a fully humanized IgG1 mAb directed toward EGFRvIII and amplified-wt-EGFR that is conjugated to monomethyl auristatin F. It is effective in ~50% of selected patients with recurrent GBM. We have verified that GBM CSLCs are sensitive to ABT-414 and assembled a well-characterized panel of GBM patient-derived xenografts (PDX) that show a range of sensitivities to the therapy in vitro and in vivo. We also have verified, in vitro and in vivo, that CSLCs in the perivascular space in GBM internalize ABT-414, and that it is trafficked by endocytic pathways to recycling compartments as well as to the lysosome, where release of the warhead occurs. We propose to manipulate two genes, RBSN and EHD3, that are associated with recycling and exhibit dysregulated expression in a proportion of GBM tumors. Preliminary data indicate that alterations in expression of these genes in CSLCs can reduce diversion of ABT-414 to the recycling compartments and enhance CSLC killing. We propose two Specific Aims in which we will determine: (1) The effects of manipulation of RBSN in GBM-derived CSLCs on the trafficking fate of ABT-414 and CSLC survival in vitro after deletion/downregulation of RBSN; blockade of endosome maturation using shVps39; and expression of tagged mutant-RBSN. The effects of RBSN deletion downregulation, and the impact on ABT-414 therapeutic efficacy, will be tested in vivo using an orthotopic GBM PDX model. (2) The effects of manipulation of EHD3 in GBM-derived CSLCs on the trafficking fate of ABT-414 and CSLC survival in vitro after deletion or overexpression of EHD3, and expression of mutant EHD3. In parallel, we will determine if hypermethylation of the EHD3 gene promoter represses EHD3 expression in GBM biopsies. The effects of overexpression of EHD3 or mutant-EHD3, and their impact on ABT-414 therapeutic efficacy, will be tested in vivo using an orthotopic GBM PDX model.